This invention relates in general to a system for measuring the distance between two locations, and in particular to a distance measuring system which operates passively at remote locations, that is, without need for a transmitter at any remote location.
The need for apparatus which can effectively measure the distance between two locations without actually bridging the locations with a measuring tape or the like is well-known in many arts. One such application for distance measuring apparatus is in the field of surveying, where distances too lengthy or inaccessible for a measuring tape have traditionally been measured either by triangulation, or by techniques which measure the transit time of radiation from a home location outbound to a remote location (which typically includes a mirror or another reflective device) and return to the home location. Triangulation techniques are typically time-consuming and, depending on the distances involved, may be relatively inaccurate. The more sophisticated surveying measurement techniques involving the measurement of radiation transit time are relatively expensive and sophisticated, particularly in the context of surveying and related field activities, inasmuch as apparatus capable of accurately measuring the transit time of light is usually required.
Another application for distance measuring systems is found in aircraft navigation. While distance-measuring equipment (DME) has been used for some time in aircraft navigation, only those aircraft equipped with a DME transponder may determine their range from a DME base station. That "range", moreover, is determined by the transit time of radio signals between the DME-equipped aircraft and a DME base station, and so the measured range is actually the line-of-sight slant range between the aircraft and the base station, rather than being the ground range which the pilot desires to know. DME transponders are relatively expensive and are actually radio transmitter-receiver combinations, and the number of DME-equipped aircraft which can simultaneously interrogate a particular base station for ranging purposes is limited by technical considerations.